1. Field of the Invention
The present invention relates to a method and apparatus for detecting synchronization at a receiving stage of an orthogonal frequency division multiplexing (OFDM) transmission system, and more particularly, to a method and apparatus of detecting a synchronization using samples quantized in 2n levels.
2. Description of the Related Art
An orthogonal frequency division multiplexing (OFDM) system used in European digital video broadcasting for television (DVB-T), digital audio broadcasting (DAB), and high-speed wireless local area network (WLAN) systems synchronize a timing by detecting an offset in the timing of a received frame using a correlation between a received frame signal and a cyclic prefix signal for a frame.
FIG. 1 shows a transmission data format used in the European DVB-T system. Transmission data 100 includes a first cycle prefix 110, data 120, a second cycle prefix 140, and data 150. The first cycle prefix 110 is a copy of a portion 130 (i.e., duplication data) of the data 120, and the second cycle prefix 140 is a copy of a portion 160 of the data 150. The term “cycle prefix” is a duplicate portion of the transmission data 100 used for a purpose of detecting synchronization. When the cycle prefix is included in a data frame at a transmitting stage, a starting point of the data can be detected at a receiving stage using the cycle prefix.
FIG. 2 is a block diagram of a conventional correlation system for detecting a synchronization of the timing. Referring to FIG. 2, the correlation system 200 includes a delaying unit 210 delaying received input data samples by N clocks, a complex conjugate extracting unit 220 extracting complex conjugates of outputs of the delaying unit 210, a multiplying unit 230 multiplying outputs of the complex conjugate extracting unit 220 by the received input data samples, an integer extracting unit 240 extracting only integer parts from outputs of the multiplying unit 230, a moving sum calculating unit 250 summing up consecutive L outputs of the integer extracting unit 240 at every clock, and a peak detecting unit 260 detecting a maximum value among outputs of the moving sum calculating unit 250 and determining the synchronization of timing.
The moving sum calculating unit 250 calculates correlation values according to the following equation:
                              Λ          ⁡                      (            n            )                          =                              ∑                          k              =              1                                      n              +              L                                ⁢                                    r              ⁡                              (                k                )                                      ⁢                                          r                *                            ⁡                              (                                  k                  -                  N                                )                                                                        (        1        )            where r(k) is a received signal sampled with a baseband frequency, N is a size of input data for Digital Fourier Transform (DFT) used in the OFDM system, and r*(k−N) is complex conjugate data of r(k) delayed by N clocks. Among the calculated correlation values Λ(n), a maximum correlation value Λmax(n) represents a correlation peak, and based upon whether and where the correlation peak is detected, the timing is synchronized.
That is, in the transmission data shown in FIG. 1, there is a time delay of N clocks between the first cycle prefix data 110 and the duplication data 130 thereof. Therefore, if the correlation system 200 shown in FIG. 2 delays the received input data samples by N clocks, the data interval between every other cycle prefix can be spaced, and if the received input data sample refers to the cycle prefix, the moving sum calculating unit 250 outputs a maximum value because the cycle prefix and the duplication data 130 thereof are the same data. In this way, the correlation system shown in FIG. 2 can detect the synchronization of timing.
FIG. 3 is a block diagram of another conventional correlation system to detect the synchronization of the timing. Referring to FIG. 3, the correlation system 300 includes a sign bit quantizing unit 310 for quantizing input data using only sign bits of the input data, i.e., quantizing input signals as +1 if the input signals are greater than zero, or otherwise, as −1, a delaying unit 320 for delaying outputs of the sign bit quantizing unit 310 by N clocks, a complex conjugate extracting unit 330 for extracting complex conjugates of outputs of the delaying unit 320, and a multiplying unit 340 for multiplying outputs of the complex conjugate extracting unit 330 by the outputs of the sign bit quantizing unit 310. The correlation system 300 also includes an integer extracting unit 350 for extracting only integer parts from outputs of the multiplying unit 340, a moving sum calculating unit 360 for summing up consecutive L outputs of the integer extracting unit 350 at every clock, and a peak detecting unit 370 for detecting a maximum value among outputs of the moving sum calculating unit 360 and determining the synchronization of timing.
The conventional correlation systems described above require multiplying units for obtaining correlation values. However, because the circuit configurations for the multiplying units are very complex, the conventional correlation systems have disadvantageously complex hardware structures.